This invention relates to reducing the effect of errors in the positioning of scan lines on an area, such as in the positioning of print lines on a media, or in the scanning of a surface to extract reflection information. This invention can apply to any raster scan device using multiple simultaneous scan lines, such as a printer having multiple printing markers, or a laser scanner having multiple scanning beams.
In the technologies of printing and scanning it is known to scan an area with a raster scan, which can be a predetermined pattern of scan lines, such as a set of closely spaced parallel lines, that provide substantially uniform coverage of an area. In printing, it is known to form a print image from a series of adjacent print lines running down the length of a page. In scanning, it is known to convert continuous tone images into "half-tone" images which are discretized along a series of scan lines. It is also known to use multiple printing markers or multiple scanning beams to achieve increased speed of printing or scanning.
The process of dividing up an area by scan lines, and in dividing sets of scan lines among multiple printing markers or multiple scanning beams, causes forms of sampling or quantized error which can directly, or through interaction with other processes, introduce errors in the scanned or printed image at frequencies that fall within the response characteristics of the human eye.
Consider for example, a simple raster scan device, a dot matrix printer with a print head holding nine marking elements, which can print a set of nine print lines across a page during each scan. After each scan, the print head is repositioned nine lines lower on the page, to print the next set of nine print lines. Ideally, the next set of print lines would begin exactly one line spacing below the previous set of print lines.
Errors in the spacing between sets of print lines can be quite noticeable and objectionable. For example, if the print head moves too far down the page, the sets are placed too far apart, and a bare unmarked strip can appear which might appear as a bright line across a dark background. Conversely, if the print head moves too little down the page, the sets are placed too close together, and an overlapped, double-printed strip can appear, which might appear as a dark line across a light background.
These, and similar types of boundary errors, can occur whenever the raster scan is built up from smaller sets of scan lines, or when the scan lines are divided among multiple markers or scanning beams. Proper spacing between sets of scan lines is needed to prevent these errors. In printing, these errors appear as visual defects in the printed image. In scanning, these errors reduce the accuracy of the scanned information as gaps or overlaps can occur.
Exacerbating these errors is the fact that they will often occur at frequencies at which the human eye is highly sensitive. For example, in known types of printers, with sets of 9 to 24 markers in the print head and for print densities and resolutions of 100 to 600 lines per inch, the spacing of these types of errors will usually fall at frequencies to which the human eye is quite sensitive, for example, roughly around one millimeter intervals. Of course, the frequency of these errors will vary depending on the number of multiple markers, marker spacing, and print density. The severity of these errors will vary depending on the type of marking technology, mechanical constraints on print head movement, and the precision and rigidity of mechanical construction and operation.
It is difficult to reduce these errors by increasing the precision of scan line positioning, for example, by finer positioning of the print head. Experiments have shown that the precision of print head positioning required for acceptable visual results is on the order of one-eighth of one print line spacing or better. This can be difficult and expensive to achieve, and difficult to maintain over the working life of a printer.
It is also difficult to reduce these errors by changing the spacing of the errors, for example, by printing in smaller or larger sets of print lines. Printing with a smaller set of print lines from a smaller print head would be quite slow since it would require many more passes to complete a full page image. Printing with a larger set of print lines from a larger print head reduces the number of times the error appears on a page, but does not reduce the visual contrast of any single error.